Multi-bit stram memory cells

ABSTRACT

A multi-bit spin torque magnetic element that has a ferromagnetic pinned layer having a pinned magnetization orientation, a non-magnetic layer, and a ferromagnetic free layer having a magnetization orientation switchable among at least four directions, the at least four directions being defined by a physical shape of the free layer. The magnetic element has at least four distinct resistance states. Magnetic elements with at least eight magnetization directions are also provided.

RELATED APPLICATIONS

This application claims priority to U.S. provisional patent applicationNo. 61/087,210, filed on Aug. 8, 2008. The entire disclosure ofapplication No. 61/087,210 is incorporated herein by reference.

BACKGROUND

Spin torque transfer technology, also referred to as spin electronics,which is based on changing magnetic state of the system by momentumtransfer from conduction electrons, is a recent development. Spin torqueRAM or ST RAM is a non-volatile random access memory application thatutilizes spin torque technology. Digital information or data,represented as a “0” or “1”, is storable in the alignment of magneticmoments within a magnetic element. The resistance of the magneticelement depends on the moment's alignment or orientation. The storedstate is read from the element by detecting the component's resistivestate.

The magnetic element, in general, includes a ferromagnetic pinned layer(PL), and a ferromagnetic free layer (FL), each having a magnetizationorientation. A non-magnetic barrier layer is therebetween. Themagnetization orientations of the free layer and the pinned layer definethe resistance of the overall magnetic element. Usually the orientationof the PL is fixed by the strong exchange coupling of anantiferromagnetic layer which is immediate contact with the PL. Theresistance is changed by changing the orientation of the FL. Oneparticular type of such an element is what is referred to as a “spintunneling junction,” “magnetic tunnel junction cell”, “spin torquememory cell”, and the like. When the magnetization orientations of thefree layer and pinned layer are parallel, the resistance of the elementis low (R_(L)). When the magnetization orientations of the free layerand the pinned layer are antiparallel, the resistance of the element ishigh (R_(H)). The magnetization orientation is switched by passing acurrent perpendicularly through the layers. The current direction isdifferent for writing “1” or ‘0”. To write “1” (R_(H)) the current flowsfrom the PL to FL, and reversed to flow from FL to PL to write “0”(R_(L)).

Many magnetic elements store only two states or data bits, i.e., “0” and“1”. Some magnetic elements, often referred to as multi-bit elements,are configured to store multiple states or data bits, i.e., four bits“00”, “01”, “10” and “11”. One configuration for a four bit multi-bitelement has two magnetic tunnel junctions combined, so that the magneticelement has two free layers and two pinned layers. The two free layershave different resistances, so that four different resistances areavailable for the overall resistance of the element. Other designs ofmulti-bit elements can be used.

BRIEF SUMMARY

The present disclosure relates to multiple-bit or multi-bit magneticelements. The magnetic elements have a free layer with a physicalconstruction configured to allow for at least four magnetizationorientations.

In one embodiment, this disclosure provides a multi-bit spin torquemagnetic element that has a ferromagnetic pinned layer having a pinnedmagnetization orientation, a non-magnetic layer, and a ferromagneticfree layer having a magnetization orientation switchable among at leastfour directions, the four directions being defined by a physical shapeof the free layer. The magnetic element has at least four distinctresistance states.

These and various other features and advantages will be apparent from areading of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing detailed description of various embodiments of the disclosurein connection with the accompanying drawings, in which:

FIG. 1A is a cross-sectional schematic diagram of an illustrativemagnetic element in a low resistance state; FIG. 1B is a cross-sectionalschematic diagram of the magnetic element in a high resistance state;

FIG. 2 is a schematic top view of a ferromagnetic free layer of amagnetic element, the free layer having four possible magnetizationorientations; FIGS. 2A through 2D show the four possible magnetizationorientations for the free layer of FIG. 2;

FIG. 3 is graphical diagram of the magnetization orientation of aferromagnetic pinned layer in relation to four magnetizationorientations of a ferromagnetic free layer;

FIGS. 4A through 4D show the four possible magnetization orientations ofthe free layer in relation to the pinned layer;

FIG. 5 is a graphical representation of the normalized energy barrierfor switching the free layer between two neighboring magnetic states

FIGS. 6A through 6D are graphical representations of results frommicromagnetic modeling of four magnetization orientations of a freelayer for switching between two neighboring states;

FIGS. 7A through 7D are additional graphical representations of resultsfrom computer modeling of four magnetization orientations of a freelayer;

FIG. 8 is a schematic top view of a ferromagnetic free layer of amagnetic element, the free layer having four possible magnetizationorientations;

FIG. 9 is a schematic top view of a ferromagnetic free layer of amagnetic element, the free layer having eight possible magnetizationorientations; and

FIG. 10 is a schematic diagram of a multi-bit memory cell utilizing amagnetic element.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanying setof drawings that form a part hereof and in which are shown by way ofillustration several specific embodiments. It is to be understood thatother embodiments are contemplated and may be made without departingfrom the scope or spirit of the present disclosure. The followingdetailed description, therefore, is not to be taken in a limiting sense.The definitions provided herein are to facilitate understanding ofcertain terms used frequently herein and are not meant to limit thescope of the present disclosure.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” encompass embodiments having pluralreferents, unless the content clearly dictates otherwise. As used inthis specification and the appended claims, the term “or” is generallyemployed in its sense including “and/or” unless the content clearlydictates otherwise.

The present disclosure relates to multi-bit magnetic elements where thefree layer has a shape configured to hold at least four magnetizationorientations. In accordance with the invention of this disclosure, thereare at least four different relative orientations between the free layermagnetization orientation and the pinned layer magnetizationorientation, due to the physical shape of the free layer. In someembodiments, there are four different relative orientations, providingfour different data states for a two-bit magnetic element. In some otherembodiments, there are eight different relative orientations between thefree layer magnetization orientation and the pinned layer magnetizationorientation, providing eight different data states for a three-bitmagnetic element. Other embodiments may provide sixteen differentrelative orientations for a four-bit magnetic element. Having a freelayer with a physical construction configured to allow for at least fourmagnetization orientations allows for forming multi-bit magneticelements without the need for two free layers in the magnetic element,thus allowing for smaller dimensions.

While the present disclosure is not so limited, an appreciation ofvarious aspects of the disclosure will be gained through a discussion ofthe examples provided below.

FIGS. 1A and 1B are a cross-sectional schematic diagram of a genericmagnetic element 10; in FIG. 1A, element 10 is in the low resistancestate, with the magnetization orientations parallel and in FIG. 1B,element 10 is in the high resistance state, with the magnetizationorientations anti-parallel. Magnetic element 10 may also be referred toas a variable resistive memory cell or variable resistance memory cellor the like.

Magnetic element 10 includes a ferromagnetic free layer 12 and aferromagnetic reference (i.e., pinned) layer 14. Ferromagnetic freelayer 12 and ferromagnetic pinned layer 14 are separated by anon-magnetic layer 13. Proximate ferromagnetic pinned layer 14 is anantiferromagnetic (AFM) pinning layer 15, which pins the magnetizationorientation of ferromagnetic pinned layer 14 by exchange bias with theantiferromagnetically ordered material. Examples of suitable pinningmaterials include PtMn, IrMn, and others. Note that other layers, suchas seed or capping layers, are not depicted for clarity.

Ferromagnetic layers 12, 14 may be made of any useful ferromagnetic (FM)material such as, for example, Fe, Co or Ni and alloys thereof, such asNiFe and CoFe, and ternary alloys, such as CoFeB. Either or both of freelayer 12 and pinned layer 14 may be either a single layer or anunbalanced synthetic antiferromagnetic (SAF) coupled structure, i.e.,two ferromagnetic sublayers separated by a metallic spacer, such as Ruor Cu, with the magnetization orientations of the sublayers in oppositedirections to provide a net magnetization. Either or both layer 12, 14are often about 0.1-10 nm thick, depending on the material and thedesired resistance and switchability of free layer 12.

If magnetic element 10 is a magnetic tunnel junction cell, non-magneticlayer 13 is an insulating barrier layer sufficiently thin to allowtunneling of charge carriers between pinned layer 14 and free layer 12.Examples of suitable electrically insulating material include oxidematerials (e.g., Al₂O₃, TiO_(x) or MgO). If magnetic element 10 is aspin-valve cell, non-magnetic layer 13 is a conductive non-magneticspacer layer. For either a magnetic tunnel junction cell or aspin-valve, non-magnetic layer 13 could optionally be patterned withfree layer 12 or with pinned layer 14, depending on process feasibilityand device reliability.

The following are various specific examples of magnetic tunnel junctioncells. In some embodiments of magnetic element 10, layer 13 is oxidebarrier Ta₂O₅ (for example, at a thickness of about 0.5 to 1 nanometer)and ferromagnetic free layer 12 and ferromagnetic pinned layer 14include NiFe, CoFe, CoFeB or Co, or combinations of them. In otherembodiments of magnetic tunnel junction cells, layer 13 is GaAs (forexample, at a thickness of about 5 to 15 nanometers) and ferromagneticfree layer 12 and ferromagnetic pinned layer 14 include Fe, bcc-Co, orbcc-CoFe. In other embodiments of magnetic tunnel junction cells, layer13 includes Al₂O₃ (for example, a few nanometers thick) andferromagnetic free layer 12 and ferromagnetic pinned layer 14 includeNiFe, CoFe, or Co. In yet other embodiments of magnetic tunnel junctioncells, layer 13 includes MgO (for example, a few nanometers thick) andferromagnetic free layer 12 and ferromagnetic pinned layer 14 includeCoFeB, CoFe, or Co. The dimensions of magnetic element 10 are small,from about 50 to about a few hundred nanometers.

Returning to FIGS. 1A and 1B, a first electrode 16 is in electricalcontact with ferromagnetic free layer 12 and a second electrode 17 is inelectrical contact with ferromagnetic pinned layer 14 via pinning layer15. Electrodes 16, 17 electrically connect ferromagnetic layers 12, 14to a control circuit providing read and write currents through layers12, 14. The resistance across magnetic element 10 is determined by therelative orientation of the magnetization vectors or magnetizationorientations of ferromagnetic layers 12, 14. The magnetization directionof ferromagnetic pinned layer 14 is pinned in a predetermined directionby antiferromagnetic pinning layer 15 while the magnetization directionof ferromagnetic free layer 12 is free to rotate under the influence ofspin torque. In accordance with the invention of this disclosure, themagnetization direction of free layer 12 is free to rotate to at leastfour different orientations.

FIG. 1A illustrates magnetic element 10 where the magnetizationorientation of ferromagnetic free layer 12 is parallel and in the samedirection of the magnetization orientation of ferromagnetic pinned layer14. FIG. 1B illustrates magnetic element 10 where the magnetizationorientation of ferromagnetic free layer 12 is anti-parallel and in theopposite direction of the magnetization orientation of ferromagneticpinned layer 14. These are two of the possible magnetizationorientations of free layer 12; additional possible magnetizationorientations are illustrated and discussed herein in reference to laterfigures.

Switching the resistance state and hence the data state of magneticelement 10 via spin-transfer occurs when a current, under the influenceof a magnetic layer of magnetic element 10, becomes spin polarized andimparts a spin torque on free layer 12 of magnetic element 10. When asufficient level of polarized current and therefore spin torque isapplied to free layer 12, the magnetization orientation of free layer 12can be changed among different directions and accordingly, magneticelement 10 can be switched between the parallel state (i.e., as in FIG.1A), the anti-parallel state (i.e., as in FIG. 1B), and other states.

The illustrative spin-transfer torque magnetic element 10 is used toconstruct a memory device where a data bit is stored in the spin torquememory cell by changing the relative magnetization state of free layer12 with respect to pinned layer 14. The stored data bit can be read outby measuring the resistance of element 10 which changes with themagnetization direction of free layer 12 relative to pinned layer 14. Inaccordance with the invention of this disclosure, there are at leastfour different relative orientations between free layer 12 and pinnedlayer 14; in some embodiments, there are four different relativeorientations, and in some other embodiments, there are eight differentrelative orientations.

In order for the spin-transfer torque magnetic element 10 to have thecharacteristics of a non-volatile random access memory, free layer 12exhibits thermal stability against random fluctuations so that theorientation of free layer 12 is changed only when it is controlled tomake such a change. This thermal stability can be achieved via themagnetic anisotropy using different methods, e.g., varying the bit size,shape, and crystalline anisotropy. Additional anisotropy can be obtainedthrough magnetic coupling to other magnetic layers either throughexchange or magnetic fields. Generally, the anisotropy causes an easyand hard axis to form in thin magnetic layers. The hard and easy axesare defined by the magnitude of the external energy, usually in the formof a magnetic field, needed to fully rotate (saturate) the direction ofthe magnetization in that direction, with the hard axis requiring ahigher saturation magnetic field.

FIG. 2 illustrates a ferromagnetic free layer having a physical shapeconfigured to provide four magnetization orientations. Free layer 20, inthe illustrated embodiment, is cross-shaped with four arms 22 extendingfrom a center section 23. Each arm 22 has a width W and a length L. Eacharm 22 has a constant width W along its length L; in other embodiments,the arms may taper or have some other shape. For free layer 20, each arm22 is the same, having the same width W and length L. In someembodiments, including illustrated free layer 20, for each arm 22, itswidth W is the same as its length L. In other embodiments, width W isgreater than length L, and in other embodiments, width W is less thanlength L. Examples of suitable lengths L include about 1-100 nm;examples of suitable widths W include about 1-100 nm. In someembodiments, either or both length L and width W are about 2-20 nm. Inone exemplary embodiment, length L and width W are about ten times (10×)the thickness of free layer 20.

Arms 22 meet at interior corners 25. As described above in reference tomagnetic element 10, free layer 20 is a ferromagnetic material such as,for example, Fe, Co or Ni and alloys thereof, such as NiFe and CoFe, andternary alloys, such as CoFeB. Arms 22 and interior corners 25 may beformed, for example, by removing portions of a previously depositedferromagnetic material layer, for example, by ion milling or wetmilling. In alternate embodiments, arms 22 and interior corners 25 maybe formed, for example, by depositing ferromagnetic material using amask.

Free layer 20 has four stable magnetization orientations, illustrated inFIGS. 2A through 2D. In this embodiment of a shaped free layer, the fouraverage magnetization orientations are toward the four interior corners25, between adjacent arms and orthogonal to each other. In FIG. 2A, themagnetization orientation is toward a first corner 25A; in FIG. 2B, themagnetization orientation is toward a second corner 25B that is 90degrees from first corner 25A; in FIG. 2C, the magnetization orientationis toward a third corner 25C, which is 180 degrees from first corner 25Aand 90 degrees from second corner 25B; and in FIG. 2D, the magnetizationorientation is toward a fourth corner 25D, which is 270 degrees (or −90degrees) from first corner 25A, 180 degrees from second corner 25B, and90 degrees from third corner 25C. Adjacent orientations are 90 degreesapart. Opposite orientations (e.g., 25A, 25C and 25B, 25D) are the easyaxes of free layer 20.

For embodiments where the overall lateral dimension (e.g. length) offree layer 20 is less than about 100 nm, a strong exchange couplinginteraction within free layer 20 maintains a state close to a singledomain state for free layer 20. (An overall lateral dimension of 100 nmcan be obtained with each arm 22 of free layer 20 having a length L anda width W of about 33.3 nm.) Free layer 20 can be engineered to have lowmagneto crystalline anisotropy and to have isotropic sheet filmproperties. These properties, in a four-fold symmetry geometry, createfour stable magnetization orientations, as illustrated in FIGS. 2Athrough 2D. These four stable magnetization orientations can be utilizedto create four different resistance states for two-bit per cellnon-volatile memory.

The four magnetization orientations are in relation to the magnetizationorientation of the corresponding pinned layer. FIG. 3 illustratesgraphically the four magnetization orientations 30A, 30B, 30C, 30D offree layer 20. A reference line 32 is provided, which corresponds to oneof arms 22 of free layer 20. The magnetization orientation of thecorresponding pinned layer is labeled 35 and is present at an angle αfrom reference line 32. In order to have four distinct resistancestates, the magnetization orientation of the pinning layer is neitheraligned with reference line 32 (nor any of arms 22) nor themagnetization orientations 30A, 30B, 30C, 30D (i.e., not aligned withinterior corners 25A, 25B, 25C, 25D of free layer 20). The magnetizationorientation of the pinning layer can be at an angle α of about 10-35degrees from reference line 32. In one embodiment, the magnetizationorientation angle α is 22.3 degrees from reference line 32; in anotherembodiment, the magnetization orientation is equally between reference32 and magnetization orientation 30A, having an angle α of 22.5 degreesfrom reference line 32.

Switching between the four distinct orientations, and thus fourdistinct, resistance states, can be achieved via spin torque effects.FIGS. 4A through 4D illustrate the four resistance states, due to apinned layer magnetization orientation 45 with the four possible freelayer magnetization orientations 40A, 40B, 40C, 40D. Reference line 42is provided. In FIGS. 4A through 4D, the actual free layer magnetizationorientation is identified as 48.

FIG. 4A illustrates a first state, State 1, with magnetizationorientation 48 oriented to direction 40A, that can be achieved bydriving a large current (for example, from about 100 microAmps toseveral hundred microAmps) from the pinned layer to the free layer inthe magnetic element, no matter in which direction the magnetizationorientation is originally oriented. State 2, in FIG. 4B, is obtained bypassing a current from the free layer of State 1 (FIG. 4A) to the pinnedlayer. The current is selected to be sufficient to switch magnetizationorientation 48 to direction 40B but not too large to switchmagnetization orientation 48 to direction 40C. To switch to State 3 inFIG. 4C from State 2, a current is passed from the free layer to thepinned layer. To switch to State 4 in FIG. 4D from State 3, current ispassed from the pinned layer to the free layer. The current is selectedto be sufficient to switch magnetization orientation 48 to direction 40Dbut not too large to switch magnetization orientation 48 to direction40A (State 1).

FIG. 5 graphically illustrates an exemplary normalized energy barrierfor switching the magnetization orientation 90 degrees between twoadjacent states, for example, from direction 40B to direction 40A. FIG.5 provides the normalized magnetic energy at various points betweendirection 40B and direction 40A to switch the magnetization orientationfrom direction 40B to direction 40A.

FIGS. 6A through 6D and FIGS. 7A through 7D graphically illustrate fourmagnetization orientations, taken at four different points, for anexemplary ferromagnetic free layer as its magnetization orientation isswitched from direction 40B to direction 40A (of FIGS. 4A through 4D).In particular, FIGS. 6A and 7A show the magnetization orientation atpoint 0 of FIG. 5 (i.e., the magnetization orientation is towarddirection 40B), FIGS. 6B and 7B show the magnetization orientation atpoint 10 of FIG. 5, FIGS. 6C and 7C show the magnetization orientationat point 15 of FIG. 5, and FIGS. 6D and 7D show the magnetizationorientation at point 23 of FIG. 5 (i.e., the magnetization orientationis now toward direction 40A).

In order to obtain a thermally stabile magnetic tunnel junction, anormalized energy barrier (ΔE/k_(B)T) of at least about 45 is desired.It has been found that a free layer having a cross shape, e.g., such asfree layer 20 of FIG. 2, with a layer thickness of about 2-3 nm, a crossdimension of about 100 nm and less, and a magnetization M_(s) of about1200 emu/cc, provide a generally inferior design than a cross shapedfree layer with a thickness of 3 nm or greater, a cross dimension ofabout 100 nm or greater, and magnetization M_(s) of about 1400 emu/cc.This was determined based on computer modeling of free layers usingvarious parameters.

The graphs of FIGS. 6A through 6D were modeled using a free layer suchas free layer 20 of FIG. 2, using two different sets of parameters. Thefirst set of parameters set a free layer magnetization of M_(s)=1200emu/cc, a cross-shape having length L of 25 nm and width W of 25 nm(providing an overall dimension of 75 nm), a layer thickness of 2.5 nm,and exchange coupling length l_(ex)=8.8 nm. The resulting energy barrierwas

$\frac{\Delta \; E}{k_{B}T} = 20$

The second set of parameters set a free layer magnetization ofM_(s)=1400 emu/cc, a cross-shape having length L of 30 nm and width W of30 nm (providing an overall dimension of 108 nm), a layer thickness of 3nm, and exchange coupling length l_(ex)=10.4 nm. The resulting energybarrier was

$\frac{\Delta \; E}{k_{B}T} = 48$

The second set of parameters provided a better configuration than thefirst set of parameters.

The graphs of FIGS. 7A through 7D were modeled using a free layer suchas free layer 20 of FIG. 2, using two different sets of parameters; theparameters for FIGS. 7A through 7D utilized a weaker exchange. The firstset of parameters set a free layer magnetization of M_(s)=1200 emu/cc, across-shape having length L of 25 nm and width W of 25 nm (providing anoverall dimension of 75 nm), a layer thickness of 2.5 nm, and exchangecoupling length l_(ex)=5.0 nm. The resulting energy barrier was

$\frac{\Delta \; E}{k_{B}T} = 23$

The second set of parameters set a free layer magnetization ofM_(s)=1400 emu/cc, a cross-shape having length L of 30 nm and width W of30 nm (providing an overall dimension of 108 nm), a layer thickness of 3nm, exchange coupling length l_(ex)=6.0 nm. The resulting energy barrierwas

$\frac{\Delta \; E}{k_{B}T} = 53$

The second set of parameters provided a better configuration than thefirst set of parameters.

Returning to FIG. 2 and FIGS. 2A through 2D, free layer 20 having fourevenly spaced arms 22 has four unevenly or unequally spaced resistancestates, due to the nonlinear dependence of the GMR effect on the on theangle between the free layer magnetization orientation and the pinnedlayer magnetization orientation. Having unequally or unevenly spacedresistance states may be a potential source for reduced reliability andpossibly errors. An alternate to the symmetric shape of free layer 20 isto have an asymmetric or uneven free layer shape designed to provideequidistant resistance levels.

FIG. 8 illustrates an embodiment of a ferromagnetic free layer formulti-bit magnetic elements, the free layer having an asymmetric shapewith four equidistant resistance levels. That is, the free layer isphysically not symmetrical among the four arms but the change inresistance between the possible magnetization orientations is the same.The various features of this free layer are similar to those of freelayer 20 of FIG. 2 unless otherwise indicated. Free layer 80, in theillustrated embodiment, has four arms 82 extending from a center section83. Each arm 82 has a width W and a length L. For free layer 80,adjacent arms 82 do not have the same width W and length L, but rather,adjacent arms 82 are different in width W. In some embodiments,oppositely positioned arms 82 or alternating arms 82 may have the samewidth W and/or length L. Additionally, each arm 82 increases in width Walong its length L from center section 83. Examples of suitable lengthsL include about 1-100 nm, e.g., about 5-30 nm; examples of suitablewidths W include about 1-100 nm, e.g., about 5-50 nm. Between adjacentarms 82 is a void 84.

Free layer 80 has four stabilize magnetization orientations, illustratedas orientations 85A, 85B, 85C, 85D, which provide four distinctresistance states. In this embodiment of a shaped free layer, the fourmagnetization orientations are present in void 84 between adjacent arms82. In most embodiments, the magnetization orientations are equidistantbetween two adjacent arms 82. In this asymmetric embodiment,magnetization orientations 85A, 85B, 85C, 85D are not orthogonal to eachother, but rather have an angle either less than or greater than 90degrees therebetween. Opposite magnetization orientations (e.g., 85A,85C and 85B, 85D) are 180 degrees apart; opposite magnetizationorientations are the easy axes of free layer 80. In FIG. 8,magnetization orientations 85A, 85B have an angle β therebetween. In oneexemplary embodiment, angle β is about 80.5 degrees; thus, orientations85A, 85B and 85C, 85D are 70.5 degrees apart and orientation 85B, 85Cand 85D, 85A are 109.5 degrees apart. Having angle β approximately 80degrees provides four distinct and equidistantly spaced resistances.With properly shaped asymmetric free layer, four-fold symmetry can beobtained where the two easy axes are not perpendicular but aspecifically chosen angle.

The multi-resistance (i.e., at least four) principal can be expanded tothree bits per cell by further modifying the free layer of the magneticcell and creating an eight-fold in plane symmetry with eight degenerateresistance states. FIG. 9 illustrates another embodiment of aferromagnetic free layer for multi-bit magnetic elements, the free layerhaving an asymmetric shape with eight equidistant resistance levels.Free layer 90, in the illustrated embodiment, has eight arms 92extending from a center section 93. Between adjacent arms 92 is a void94. The various features of free layer 90 are similar to those of freelayer 80 of FIG. 8 unless otherwise indicated.

Free layer 90 has eight stable magnetization orientations, illustratedas orientations 95A, 95B, 95C, 95D, 95E, 95F, 95G, 95H which provideeight distinct resistance states. The eight magnetization orientationsare present in void 94 between adjacent arms 92. In this embodiment,magnetization orientations 95A, 95B, 95C, 95D, 95E, 95F, 95G, 95H havean angle either less than or greater than 45 degrees therebetween.Opposite magnetization orientations (e.g., 95A, 95C and 95B, 95D, etc.)are 180 degrees apart; opposite magnetization orientations are the easyaxes of free layer 90. With a properly shaped asymmetric free layer,eight-fold symmetry can be obtained where the four easy axes are not at45 degrees but at a specifically chosen angle. It is noted that for sucha configuration for free layer 90 to be practical, extremely tightprocess control on shape, resistance and critical current distributionswould have to be achieved.

A multi-resistance (i.e., at least four resistance) free layer (e.g.,free layer 20, free layer 80, free layer 90) would be incorporated intoa magnetic element (e.g., a magnetic tunnel junction cell or a magneticspin valve) such as magnetic element 10 of FIGS. 1A and 1B, having acorresponding pinned layer and non-magnetic layer between the free layerand the pinned layer. The magnetic element could then be incorporatedinto a magnetic memory unit. FIG. 10 shows a multi-bit magnetic elementintegrated with a transistor in a typical spin torque memory (i.e., STRAM).

Memory 100 of FIG. 10 includes a magnetic element 110 (e.g., magneticelement 10 having free layer 20, free layer 80 or free layer 90).Magnetic element 110 is electrically connected to a bit line BL and asource line SL through a series of connections. Proximate element 110 isa first electrode 111 that is electrically connected to source line SLby a first via 101, a second via 102, a third via 103 and a fourth via104. Positioned between the vias are conductive interconnects 106, 107,108, which together with vias 101, 102, 103, 104 provide electricalconnection from magnetic element 110, through gate 109 to source lineSL. Proximate element 110, opposite first electrode 111, is a secondelectrode 112 that is electrically connected to bit line BL by fifth via105.

A controller, not illustrated, applies current through magnetic element110 to switch the magnetization orientation of the free layer and thusswitch the resistance state. Depending on the amplitude of the current,the magnetization orientation and thus resistance state will be one ofat least four resistance states (i.e., one of four resistance states offree layer 20, 80 or one of eight resistance states of free layer 90,etc.). To read or determine the resistance state (i.e., the bit) withinmagnetic element 110, a lesser current is passed through magneticelement (typically few tens of microAmps).

The free layers, magnetic elements, and spin torque memory structures ofthis disclosure may be made by well-known thin film building and removaltechniques such as chemical vapor deposition (CVD), physical vapordeposition (PVD), atomic layer deposition (ALD), photolithography, dryetching, wet etching, or ion milling. The physical shape of the freelayer will affect the method for producing the shape. For example, afree layer having four arms (for example, such as free layer 20) may beeasiest to make by deposition. Whereas, for example, a free layer havingnumerous arms (for example, such as free layer 90 having eight arms 92)may be easier made by ion milling than by deposition, but could be madeby deposition.

Thus, embodiments of the MULTI-BIT STRAM MEMORY CELLS are disclosed. Theimplementations described above and other implementations are within thescope of the following claims. One skilled in the art will appreciatethat the present disclosure can be practiced with embodiments other thanthose disclosed. The disclosed embodiments are presented for purposes ofillustration and not limitation, and the present invention is limitedonly by the claims that follow.

1. A multi-bit spin torque magnetic element comprising: a ferromagneticpinned layer having a pinned magnetization orientation; a ferromagneticfree layer having a magnetization orientation switchable among at leastfour directions, the at least four directions defined by a physicalshape of the free layer; and a non-magnetic layer between the pinnedlayer and the free layer, the magnetic element having at least fourdistinct resistance states.
 2. The magnetic element of claim 1, whereinthe at least four directions are 90 degrees apart.
 3. The magneticelement of claim 1, wherein the at least four directions are not 90degrees apart.
 4. The magnetic element of claim 1 wherein the free layerhas a magnetization orientation switchable among at least eightdirections, the eight directions defined by the physical shape of thefree layer, and the magnetic element having at least eight distinctresistance states.
 5. A multi-bit spin torque magnetic elementcomprising: a ferromagnetic pinned layer having a pinned magnetizationorientation; a ferromagnetic free layer having a magnetizationorientation switchable among at least four directions, the free layerhaving a physical shape with at least four arms, with one of the fourdirections extending between adjacent arms; and a non-magnetic layerbetween the pinned layer and the free layer.
 6. The magnetic element ofclaim 5, wherein each of the at least four arms has a length and awidth, and wherein each of the at least four arms has the same length aswidth.
 7. The magnetic element of claim 5, wherein each of the at leastfour arms has a length and a width, and wherein each of the at leastfour arms has the same length and width.
 8. The magnetic element ofclaim 5, wherein each of the at least four arms has a length and a widththat are the same, and wherein each of the at least four arms has thesame length and width.
 9. The magnetic element of claim 5, wherein eachof the at least four arms has a length and a width, and wherein thewidth of each of the at least four arms changes along the length of thearm.
 10. The magnetic element of claim 5, wherein the at least fourdirections are 90 degrees apart.
 11. The magnetic element of claim 5,wherein the at least four directions are not 90 degrees apart.
 12. Themagnetic element of claim 5 wherein the free layer has a magnetizationorientation switchable among at least eight directions, the free layerhas a physical shape with at least eight arms, with one of the eightdirections extending between adjacent arms.
 13. The magnetic element ofclaim 12, wherein each of the at least eight arms has a length and awidth, and wherein the width of each of the at least eight arms changesalong the length of the arm.
 14. The magnetic element of claim 5,wherein the pinned layer magnetization orientation is between twoadjacent of the at least four directions.